1. Field of the Invention
The present invention relates to a large scale integrated circuit (LSI) device having a barrier layer interposed between a wiring conductor layer and a substrate and a method and an apparatus used for manufacturing such an LSI device.
2. Description of the Related Art
In the large scale integrated circuit or LSI devices known heretofore, there is provided at an interface between a wiring conductor layer and an insulation layer a barrier layer formed of titanium nitride or the like with a view to preventing a material of the wiring conductor layer from being diffused into the insulation layer and at the same time for ensuring a positive adhesion between the wiring conductor layer and the insulation layer. As the LSI device tends to be implemented with a higher integration density in a finer structure or configuration, thickness of the individual layers or films constituting the LSI device decreases more and more, resulting in that the quality of a layer of film is more susceptible to the influence of an underlying layer or film. Under the circumstances, there exists a demand for disposition of the barrier layer of a high quality.
Besides, in accompanying an increase in the integration density of the LSI device, the design rule therefor (such as a width of the wiring conductor layer) becomes fine. For example, in the case of a 16 M-bit DRAM (Dynamic Random Access Memory), the rule for the conductor width or the like is less than half a micron. Further, contact holes formed along an aluminum wiring film or layer are each of 0.5 .mu.m in diameter with an aspect ratio greater than "2" in the case of a 64 M-bit DRAM, while in the case of a 256 M-bit DRAM, the diameter of the contact hole is 0.3 .mu.m with the aspect ratio being on the order of "5". Besides, in the case of a 1 G-bit DRAM which is expected to be developed in the not far distant future, more severe dimensional tolerances or requirements will be imposed in respect to the diameter and the aspect ratio of the contact holes. As a consequence, a higher reliability has to be realized for suppressing breakage due to a migration phenomenon. In this connection, it is noted that even in the present state of the art, difficulty is encountered in burying a wiring conductor layer within the contact hole in succession to formation of a barrier layer through evaporation or vapor deposition process, making it difficult to form a metal wiring layer smoothly in a facilitated manner.
For a better understanding of the present invention, the related art will be discussed in some detail.
FIG. 27 shows in an enlarged view an integrated circuit known heretofore. Reference may be made to "NIKKEI MICRODEVICE", (February 1991). More specifically, a multi-layer wiring section of a 64 M-bit DRAM of simple stack type cell structure is shown in a fragmental enlarged view. Referring to the figure, the integrated circuit includes a second metal wiring layer 1 formed of an alloy of aluminum, silicon and copper and/or the like, a second inter-layer insulation layer 2 of silicon dioxide, a second inter-layer interconnection plug 3, a second barrier layer 4 of titanium nitride, a first metal wiring layer 5 of aluminum, silicon or an alloy thereof, a first inter-layer insulation layer of silicon dioxide, a first inter-layer interconnection plug 7, a first barrier layer 8 of titanium nitride and a silicon substrate 9.
FIG. 28 shows in an enlarged sectional view an integrated circuit structure known heretofore. More specifically, a wiring contact hole of a 64 M-bit DRAM of a simple stack type cell structure and a layer structure thereof are shown. Referring to the figure, this known integrated circuit device is composed of a metal wiring layer 1A formed of an alloy of aluminum, silicon and copper, a barrier layer 2A of titanium nitride, a silicon substrate 3A and a contact hole 4A.
In the integrated circuit devices mentioned above, compound thin films such as of silicon dioxide (SiO.sub.2), alumina (Al.sub.2 O.sub.3), titanium nitride (TiN.sub.x) are deposited through a vacuum evaporation process, a sputtering process or a CVD (Chemical Vapor Deposition) process. However, the compound thin films deposited through the processes mentioned above suffer not a few shortcomings such as significant variance in the characteristics, inappropriateness in composition, feeble adherence and others. Besides, limitation is encountered in coating of such portions as contact holes, through-holes or the like with a layer of a uniform thickness.
FIG. 29 is a diagram showing schematically a general structure of a magnetron type sputtering apparatus which is disclosed in "Journal of Vacuum Science and Technology A", Vol. 3, No. 2, (1985). Referring to the figure, a reference numeral 25 denotes a vacuum sputtering chamber within which a sputter processing is carried out. Disposed within the vacuum chamber 25 in opposition to substrates 20 is a target 21 made of titanium and disposed on a target holder 24 in which magnets are accommodated. A shutter 22 is interposed between the substrates 20 and the titanium target 21. An inactive gas such as argon or the like is introduced into the sputtering chamber 25 through an appropriate gas supply system 23. A reference numeral 26 denotes an evacuation system for evacuating the chamber 25.
Operation of the magnetron type sputtering apparatus of the structure shown in FIG. 29 will be described below. First, the sputtering chamber 25 is evacuated through the evacuating system 26. Subsequently, nitrogen and argon gases are introduced into the chamber 25 through the gas supply system 23. Next, a bias voltage is applied to the substrates 20 with a positive polarity relative to the target 21 for the purpose of coating the substrate 20 with a barrier film for a wiring conductor layer. In this state, plasma is generated between the target 21 and the substrate 20.
At that time, electrons within the plasma are caused to rotate spirally under the magnetic forces of the magnets accommodated within the target holder 24, whereby plasma generation is much promoted.
On the other hand, argon ions in the plasma are caused to impinge on the target 21 under acceleration by the bias voltage to thereby sputter the target material or titanium. By opening the shutter 22, titanium atoms as sputtered are deposited over the substrate 20 at the presence of a nitrogen gas atmosphere to form a titanium nitride barrier film for the wiring conductors.
FIG. 30 is a sectional view showing a contact hole having an aspect ratio of about 0.8 in the state deposited with a film by using a magnetron type sputtering apparatus, which is reported in "A Collection of Preliminaries for Semicon Japan 88 Technical Symposium".
In FIG. 30, a substrate 20 is formed with contact holes 31 each having a diameter of 0.5 micron and a depth of 2.0 microns. A wiring conductor film 32 of aluminum is formed over the substrate 20 inclusive of the contact hole 31, wherein a nitride titanium barrier film 33 is formed underneath the aluminum film 32.
As can be seen in FIG. 30, the thickness of the barrier film 33 is decreased at a lower side wall portion and a bottom of the contact hole 31 as compared with that of the barrier film portion deposited at other flat portions due to growth of an overhang and a self shadowing effect taking place spontaneously in the course of processing.
In this manner, when the aluminum wiring conductor film 32 is formed over the barrier film 33 by the magnetron type sputtering process, an overhang portion is inevitably formed, resulting in reduction in the overall film thickness at the bottom of the contact hole.
As is appreciated from the above description, it is difficult to form a film in a uniform thickness over the whole surface of a contact hole having a high aspect ratio typified by a diameter of 0.5 .mu.m and a depth of 2.0 .mu.m by the wiring conductor layer formation technique which relies on the magnetron type sputtering process.
At this juncture, it is noted that in the magnetron type sputtering apparatus known heretofore, sputtered particles collide against active particles of plasma to be thereby scattered. Consequently, the particles sputtered from the target have a short mean free path on the order of several centimeters.
As will now be understood from the above, the magnetron type sputtering process suffers from a problem that probability of the sputtered particles reaching the bottom of a contact hole having an aspect ratio of a value greater than "2" is remarkably decreased, making it impossible to form a wiring conductor film or layer on offset portions such as the side wall and the bottom of the contact hole.
As the measures for coping with the problem described above, there has been developed a cluster ion beam vapor deposition apparatus in which a point-like or spot-like evaporation source is used. FIG. 31 is a schematic diagram showing as a typical one of such cluster ion beam vapor deposition apparatus a reaction type cluster ion beam (R-ICB) apparatus which is capable of forming a thin film of a relatively high quality and which is disclosed in "Proceedings of the Seventh Symposium on Ion Source and Ion Assisted Technology". Referring to FIG. 31, a vacuum or reaction chamber 41 is held at a predetermined vacuum level through an evacuation system 42 and is equipped with a reactive gas introducing system 43 which is comprised of a gas bomb filled with a reactive gas such as oxygen gas, a flow regulating valve 45 for regulating a flow of reactive gas introduced to the vacuum chamber 41 and a reactive gas introducing pipe 46.
A vapor generation source generally denoted by a numeral 50 includes a closed type crucible having a nozzle 51 and filled with a material for evaporation, a filament 53 for heating the crucible 52 and a heat shielding enclosure 54. A cluster (i.e., aggregation of atoms) 66 is formed by ejecting vapor of the material 55 for evaporation from the crucible 52 through the nozzle 51.
An ionizing means 60 for ionizing the cluster 56 is composed of an electron beam emission filament 61, an electron drawing electrode 62 for drawing and accelerating electrons emitted from the filament 61 and a heat shielding plate 63. The ionized clusters 66 are caused to collide against a substrate 67 under the acceleration effect of an electric field generated by an accelerating electrode 65. The substrate 67 has a surface formed with a thin film of oxide and is adapted to be rotated by a rotating means 69 such as an electric motor through a rotatable shaft 68. A power supply system 80 includes DC power supply sources 81, 82 and 83 for supplying DC bias voltages and power supplies 84 and 85 for supplying currents for heating the filaments 53 and 62, respectively.
The individual bias voltage sources included in the power supply system 80 serve for such functions as described below. The first DC power supply 81 is used for biasing the crucible or pot 52 with a positive potential relative to the filament 53 so that thermal electrons emitted from the crucible heating filament 53 electrically energized by the filament heat power supply 84 collide against the crucible 52. The second DC power supply 82 is used for biasing the electron drawing electrode 62 with a positive potential relative to the filament 61 so that the thermal electrons emitted from the ionizing filament 61 heated by the filament heating power supply source 85 are drawn internally of the electrode 62. Further, the third DC power supply 83 serves to bias the electron beam drawing electrode 62 and the crucible 52 with a positive potential relative to the accelerating electrode 65 which is at the ground potential, as a result of which an electric field is formed for accelerating the cluster ions carrying positive electric charge.
Next, description will be directed to operation of the cluster ion beam evaporation apparatus.
After evacuation of the vacuum chamber 41 by means of the evacuating system 42 so that vacuum lower than 10.sup.-4 Torr prevails within the chamber 41, the flow regulating valve 45 is opened to allow a reactive gas to be supplied into the chamber 41 through the gas introducing pipe 46.
On the other hand, electrons emitted from the crucible heating filament under the effect of the electric field applied by the DC power supply 81 are caused to collide against the crucible 52 to thereby heat it until the vapor pressure within the crucible 52 has attained a level of several Torrs. Then, the evaporation material 55 within the crucible is vaporized and ejected into the vacuum space through the nozzle 51. The ejected vapor undergoes adiabatic expansion in the course of flowing through the nozzle 51 to be thereby accelerated and at the same time condensed to thereby form aggregation of atoms or atom cloud referred to as the clusters 66. The clusters 66 are then partially ionized by electrons emitted from the ionizing filament 61 to be partially transformed into cluster ions, which are further accelerated under the action of an electric field generated by the accelerating electrode 65 to collide against the substrate 67 which is rotated, together with neutral cluster not ionized. Existing in the vicinity of the substrate 67 is an oxygen gas having reactivity. Thus, reaction of the clusters of evaporation material 55 with the reactive gas takes place above and in the vicinity of the substrate 67, resulting in that a thin oxide film is deposited over the exposed surface of the substrate 67.
In general, the ionized clusters 66 ejected into the vacuum space has a long mean free path. Accordingly, probability of the clusters 66 ionized and accelerated to collide with other ionized clusters before reacting the substrate 67 is low. Thus, the clusters can arrive at the contact holes and through-holes formed in the substrate with an improved directivity.
In the cluster ion beam vapor deposition apparatus having the point-like evaporation source, as shown in FIG. 31, the kinetic energy of the ionized clusters 66 which irradiate the substrate 67 can be controlled optimally by controlling correspondingly the bias voltage applied to the accelerating electrode 65, while the amount of ions can be increased by increasing the number of electrons emitted from the ionizing filament 61, whereby the amount of clusters deposited on the bottom surface of the contact hole even of an extremely small aspect ratio can be increased. In this way, there can be formed a film of an adequate thickness over the bottom surface even of the contact hole having the small aspect ratio such as typified by a hole diameter in the range of 0.5 to 1 .mu.m. However, the film deposited on the side wall of the contact hole of the small aspect ratio is thin when compared with the film formed over the bottom surface. In particular, the cluster ion beam evaporation apparatus is poor in the capability of forming a film of a desired thickness at offset portions. In this conjunction, it is to be noted that in a region where the wiring film is thin, migration of aluminum atoms takes place due to electric field concentration and/or stress concentration, incurring possibly breakage of wiring conductor.
Further, in the case where the point-like evaporation source is disposed beneath the substrate at a position coinciding with the axis of rotation of the substrate in the cluster ion beam vapor deposition apparatus, there arise problems mentioned below.
FIG. 32 is an exaggerated view of FIG. 31 and shows major portions of the cluster ion beam vapor deposition apparatus. In FIG. 32, bottom portions of the contact holes 67a which are located at positions closer to the rotation axis 68 are designated by a symbol "B" while the side wall and bottom portions located remotely from the rotation axis 68 are designated by a symbol "A". When the cluster ion beam point-like evaporation source is disposed immediately below the rotation axis 68, a satisfactory coating efficiency can be achieved at the portions A of the contact hole because the ionized clusters can easily arrive at these portions A. However, at the portions B of the contact holes, the coating and burying efficiency are degraded because of the so-called self shadowing effect.
Further, it is noted that in the above-mentioned apparatus, the reactive gas within the vacuum chamber is in the molecular state and exhibits a low activity and hence a low reaction efficiency is resulted. Consequently, a major part of the reactive gas will be discharged with the result that only an extremely small part of the reactive gas can play a role in the formation of a thin film, giving rise to a serious problem.
In an effort to solve the problem, there is proposed an approach for activating the reactive gas by excitation, dissociation and partial ionization to thereby enhance the reaction efficiency.
A compound thin film forming apparatus which is designed to serve for this end is disclosed in Japanese Patent Publication No. 11662/1988. As is shown in FIG. 33, the compound film forming apparatus known heretofore includes a gas ion source juxtaposed to an ICB (Ionized Cluster Beam) source. Parenthetically, the ICB source is of a same structure as that shown in FIG. 31. Accordingly, components thereof are designated by like reference symbols as those used in FIG. 31 and repeated description is omitted. The gas ion source 89 of the thin film forming apparatus shown in FIG. 33 includes a gas ejection nozzle 90, an electron beam emitting means 92 provided on a path of a reactive gas, an electron beam drawing electrode 91 for drawing the electron beam, an acceleration electrode 93 for accelerating the gas ions, and an inner enclosure 94 for closing substantially completely the gas ion source 89. The gas ejection nozzle 90 is connected to a gas bomb 96 through a gas conduit 95 in which a gas flow regulating valve 97 is mounted. A power supply system 100 for the gas ion source 89 includes a power supply source 101 for heating a filament which constitutes the electron beam emitting means 92, a DC power supply source 102 for biasing the electron beam drawing electrode 91 at a positive potential relative to the filament 92, and a DC power supply source 103 for biasing the electron beam drawing electrode at a positive potential relative to the accelerating electrode 93.
Operation of the compound thin film forming apparatus is performed as follows.
The vacuum chamber 41 held at a high vacuum level by the evacuating apparatus 42 is supplied with a reactive gas from the gas bomb 96 and introduced in the chamber 41 through the gas ejection nozzle 90 by adjusting the gas flow by means of the regulating valve 97 so that a gas pressure in a range of 10.sup.-4 to 10.sup.-3 Torr prevails within the vacuum chamber 41. At that time, the gas pressure within the inner enclosure 94 is held at a higher level. On the other hand, the filament 92 which functions as the electron beam emitting means is electrically heated by the heating power supply source 101, while the electron beam drawing electrode 91 disposed at a position down stream of the gas ejection nozzle 90 is applied with a bias voltage from the DC power supply source 102, whereby the reactive gas is excited, dissociated and partially ionized by the thermal electrons. Thus, the reactive gas assumes the activated state.
Subsequently, the crucible 52 is heated by the crucible heating filament 53 up to such a temperature that the vapor pressure within the crucible 52 becomes several Torrs. As a result of this, the evaporation material 55 is vaporized and ejected through the nozzle 51. The vaporized particles or clusters are partially ionized by electrons emitted from the ionizing filament 61 and accelerated under the effect of the electric field produced by the accelerating electrode 3 to be thereby forced to collide against the substrate 67 together with the vaporized particles or clusters not ionized.
On the other hand, there exists the reactive gas excited, dissociated or ionized in the vicinity of the substrate 67, and collision of the reactive gas with the vapor particles or clusters takes places to promote the reaction for depositing a chemical compound over the substrate 67 to thereby form a thin film on the substrate. When the bias voltages are applied to the accelerating electrodes 65 and 93 from the associated DC power supply sources, the aforementioned ions can arrive at the substrate 67 under acceleration. Thus, by varying the accelerating voltage, it becomes possible to control the kinetic energy of the vapor or clusters independently from that of the reactive gas ions impinging onto the substrate 67, whereby characteristics of the compound thin film formed on the substrate 67 such physical properties or quantities and structure (monocrystal, polycrystal, amorphous structure and others) can be controlled.
Although the thin film forming apparatus described just above can enjoy an improved reactivity when compared with that of the apparatus shown in FIG. 31, there arises a problem that difficulty is encountered in controlling the composition ratio of oxygen and nitrogen contained in a nitrogen oxide film upon formation thereof.